The SBlR Phase I project supported a research study to design a time- shared three wavelength phase modulation spectrophotometer to provide differential path length information directly, thereby eliminating the need for optical bench calibration. This follow-on SBIR Phase Il contract award supports the following objectives: 1) refine and test an algorithm for saturation in model systems; 2) refine engineering design and test instrument performance in model systems; 3) evaluate instrument performance and algorithm for saturation in human clinical studies.